language: rename maketables.go to gen.go

Change-Id: I95f7776127dfa09bffd33e89d8a7b45fbafbb5fa
Reviewed-on: https://go-review.googlesource.com/47345
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
2 files changed